1. Technical Field
The present invention relates to fuel cell systems comprising fuel cell stacks, and methods of operating fuel cell stacks and, in particular, to methods of purging fuel cell stacks.
2. Description of the Related Art
Electrochemical fuel cells convert reactants to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte, such as an ion-exchange membrane, interposed between two electrodes, namely a cathode and an anode, to form an electrode assembly. The electrode assembly is typically interposed between two electrically conductive flow field plates or separators that act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such separators typically comprise flow fields to supply reactants, such as fuel and oxidant, to the anode and the cathode, respectively, and to remove excess reactants and reaction products, such as water formed during fuel cell operation. Typically, a number of fuel cells are connected in series to form a fuel cell stack.
However, when air is used as the oxidant, nitrogen tends to crossover from the cathodes (and cathode flow fields) through the ion-exchange membrane to the anodes (and anode flow fields) during operation. As fuel is consumed in the fuel cell, the concentration of nitrogen in the anode flow fields increases, thus accumulating therein and negatively impacting the performance of the fuel cell stack.
In some methods of operation, fuel is recirculated in the anodes of the fuel cell stack by means of a recirculation device, such as a pump, compressor, blower, or fan, such that a portion of the exhausted fuel (or anode exhaust) is recirculated from the fuel outlet of the fuel cell stack to the fuel inlet of the fuel cell stack through an anode recirculation loop while a remainder portion is purged periodically, discussed in further detail below. The portion of exhausted fuel that is recirculated then mixes with fresh fuel from the fuel supply so that the mixed fuel entering the fuel cell stack is at least partially humidified without the need of a humidifier. In this case, nitrogen that crosses over from the cathodes through the ion-exchange membrane to the anodes during operation will also accumulate in the recirculation loop and negatively impacts the performance of the fuel cell stack and the efficiency of the recirculating device. In addition, contaminants from the fuel source can also build up in the anode flow fields (and/or anode recirculation loop) as fuel is consumed. As a result, most fuel cell systems contain a purge valve or purge assembly for purging excess nitrogen and other contaminants from the anode flow fields and/or recirculation loop. The purge valve is typically a solenoid purge valve, such as two-way (open-close) solenoid valve.
The anode flow fields are typically purged at predetermined time intervals. However, conventional purging methods often result in large pressure fluctuations in the anode flow fields (and/or anode recirculation loop) with each purge because the pressure builds up in the anode flow fields (and/or anode recirculation loop) when the purge valve is closed and then the pressure suddenly drops when the purge valve is opened. In addition, most two-way solenoid purge valves are sized so that they can adequately purge the largest required volume in a predicted range of operating conditions of the fuel cell stack (i.e., purge volumes that are required at high loads). However, at low loads, these purge valves are larger than required and, thus, purge more of the anode exhaust than necessary, again decreasing fuel efficiency.
As a result, there remains a need to develop improved methods of purging impurity gases from fuel cell stacks. The present invention addresses these issues and provides further related advantages.